Modern multi-speed transmissions utilize torque-transmitting mechanisms such as rotating type clutches and stationary clutches to transfer torque at various speed ratios through the transmission. Frequently, a torque-transmitting mechanism may be engaged in more than one fixed speed ratio (sometimes referred to as a gear ratio), i.e., the torque-transmitting mechanism is reused in the engagement schedule of the transmission. In these cases, a different torque capacity (also referred to as clutch capacity) is often required for the torque-transmitting mechanism in one gear ratio than in the other. For example, in a lower gear ratio, higher torque capacity is generally required at the engaged clutch than in a higher gear ratio. Torque-transmitting mechanisms must be designed to handle the maximum required torque capacity. This quality is referred to as the torque capacity of the clutch. When the ratio of the torque capacity for the torque-transmitting mechanism in the gear ratio requiring the maximum capacity divided by the torque capacity needed in the gear ratio required the minimum torque capacity exceeds about 3.0, it becomes challenging to control the apply and release of the torque-transmitting mechanism in the gear ratio with the lower torque capacity. Because the apply pressure will be about one-third or less of the apply pressure in the gear ratio requiring the maximum apply pressure, factors such as the tolerance on a return spring for the apply piston (i.e., the minimum force necessary to compress the return spring) affect the control of the torque-transmitting mechanism. For instance, the tolerance on the return spring may become a significant percentage of the total force on the apply piston necessary to move and engage the torque-transmitting mechanism in the gear ratio with the lower torque capacity. Additionally, the apply pressure on the piston may be so low in the gear ratio with the lower torque capacity that rotating shaft seals on the piston may not be sufficiently seated, producing a variable leak and thus compromising control of the torque-transmitting mechanism. Drag of the piston seal may also become a high percentage of the total force required to move the apply piston in the lower torque capacity instance. Change in the force of the return spring with stroke of the apply piston may also become a significant percentage of the total force on the torque-transmitting mechanism in the gear ratio requiring lower torque capacity. Finally, a solenoid valve regulating the pressure to engage the torque-transmitting mechanism is typically regulated at a very low pressure in the gear ratio requiring minimum torque capacity, and, therefore, the solenoid tolerance and hysteresis become a high percentage of the total pressure. This may make effective calibration of the apply pressure and, thus, the engagement of the torque-transmitting mechanism difficult.
One solution for achieving the different torque capacities required in different gear ratios at the same torque-transmitting mechanism is to use two different apply pistons having different areas with separate feed oils. Both of the pistons are used in the speed ratio requiring a higher torque capacity at the torque-transmitting mechanism and only one of the pistons is used in the speed ratio with lower torque capacity at the torque-transmitting mechanism. However, in a multi-speed transmission in which more than one torque-transmitting mechanism is likely to require such a dual area piston, an inordinate number of feed holes required in the transmission main shaft to feed the various apply pistons could necessitate an undesirable increase in shaft diameter. Also a rotating shaft seal is required for each apply piston, which may decrease transmission efficiency due to drag.